Systems for the evaluation of viewer preferences require the ingathering of various data from individual households or other locations equipped with television receivers, including particularly the times during which a given channel is turned on. Identification of the viewing locations is also useful in many cases. These data may be supplied to an evaluation center, usually via telephone lines, from monitoring units at the viewing locations; such units may be coupled with the respective television sets for the identification of the channels currently being received, e.g. by a system of the type disclosed in U.S. Pat. No. 4,425,578 to Haselwood et al. The monitoring units may further be provided with keyboards or the like by which viewers observing a given program may identify themselves so that the evaluation center can also receive information on, say, the number, the age and the sex of persons watching a program in a particular houshold. Reference in this connection may be made, for example, to German laid-open application No. 24 04 074, published Aug. 7, 1975, which shows such a keyboard in a system wherein channel-identifications and timing signals are fed after interim storage to a telephone for forwarding to a central data processor. A generally similar system is the subject matter of German patent No. 26 08 508, issued Feb. 28, 1980, according to which a channel sensor comprises a network disposed close to the television receiver and turned to its line-scanning frequency to determine whether the receiver is turned on or off; according to another arrangement disclosed in that German patent, a tuner connected to a TV antenna picks up the sounds of sequentially scanned channels and works into a comparator which also receives sound signals detected by a microphone juxtaposed with the television set, being thus able to ascertain to which channel--if any--the television set has been adjusted.